Can they plead the 5th?
Maybe. Probably they’d be found in Contempt. Certainly so if they give answers that are not germane. But if they try to give actual answers there’s the risk of saying something inaccurate and getting railroaded on an incidental process crime of Perjury.
This is muckraking, anyway; not a serious investigation. Let ‘em huff and puff.
Sure. But in my book, that makes them immediately guilty. I apply that standard across the board, towards commies, libs, nazis, jackbots, yahoos, anybody who declares the Fifth. What do you have to hide? If it was ANTIFA dressed up as MAGA (as was the predominant viewpoint here on FR for days and weeks after 1-6) then come clean with it and expose it and put it in the sunlight of truth. Why be afraid?